The importance of minor amounts of certain elements in nutrition, such as plant nutrition, is well established. It has been demonstrated that minor amounts of such elements as iron, manganese, copper, boron, zinc, cobalt and molybdenum are all important and essential to the proper physiological functioning of plants. Since such elements are present in most soils and some of such elements are present in all soils, and since the requisite nutritional amount is quite small, it might be assumed that whatever nutritional elements might be present would be adequate for nutritional needs. However, even where a soil contains sufficient amounts of zinc and other nutrient elements, deficiencies of these elements still occur in plants growing in that soil due to chemical and physiological processes which render such elements unavailable to the plants or make them inactive in the physiological functions within the plant. Moreover, actual iron deficiency is fairly widespread in soil which is alkaline or calcareous, such as in soil roughly west of the Mississippi River, and in sandy soil such as is found in Florida.
Still further, soil conditions can affect assimilation by a plant of zinc and other nutrient elements that may be present even in adequate amounts. In particular, an alkaline soil, that is, one having an alkaline pH, is generally regarded as being more difficult from which to supply nutritional elements for plant life than from an acidic soil. For example, iron deficiency causes chlorosis in plants in which normally green leaves turn white. Lime, which is often added to soils to reduce acidity, can induce chlorosis in plants. This is thought to be due to a high calcium carbonate content of the soil which raises the pH value of the soil to such an extent that iron is precipitated and made unavailable to plants. Still other causes become important under certain conditions.
In order to combat such soil deficiencies, it has been proposed to incorporate salts of the nutrient elements into the soil. This use has been generally unsatisfactory and of limited application, since conditions existing in the soil that caused the original deficiencies still exist. To combat the influence of disturbing soil factors, resort has also been made to spraying or injection salts of the nutrient elements. Injections of the salts can generally be applied to only relatively large plants such as trees and, while generally effective on some trees, may give rise to gumming of stone fruits and is tedious to carry out. Sprays are also cumbersome and at times unsatisfactory since they are damaging at high or otherwise effective concentrations.
A much more successful technique resides in preparing fritted glasses containing nutrient elements which are added directly to the soil. Under wet or damp growing conditions, small amounts of the nutrient element are made available in the immediate vicinity of seeds and plant roots. However, it is not a simple matter to fabricate a glass for this purpose. For example, the solubility of the glass in water should be relatively small in order to prevent a quick release of nutrient elements and also to prevent them from being rendered unavailable to the plants through chemical reaction in the soil. Yet the solubility cannot be so small that the rate of release of the nutrients from the glass is inadequate for plant growth. Further, the glass must be non-toxic in high concentrations to plants or seeds, so that large amounts of the glass can be applied at one time to furnish an ample supply of the nutrient in the soil over an extended period of time. One successful fritted glass composition for this purpose is disclosed in U.S. Pat. No. 2,732,290 to Vana et al. This is a silicate based glass adapted to furnish a number of nutrient elements such as iron, manganese, copper, zinc, boron, cobalt, and molybdenum.